1. Field of the Invention
The present invention relates to an optical fiber used for wavelength-division multiplexing (WDM) optical transmission, or more particularly, to a metropolitan system optical fiber and manufacturing method thereof.
2. Description of the Related Art
Conventionally, a technology for increasing a transmission capacity in optical transmission using an optical fiber has been pursued actively.
A transmission loss of an optical fiber generally reaches a minimum at a wavelength of approximately 1550 nm, and therefore it is desirable to use this wavelength band for optical transmission and a dispersion shifted optical fiber (DSF) having a zero dispersion wavelength around a wavelength of 1550 nm has been developed. This optical fiber allows optical transmission with a transmission capacity of several Gbps in a wavelength band of 1.55 μm.
Furthermore, quite vigorous research and development on wavelength-division multiplexing (WDM) optical transmission is being carried out as the technology for increasing a transmission capacity in recent years. Moreover, many investigations are also being carried out on an optical fiber preferably used for WDM optical transmission.
When an optical fiber is used for WDM optical transmission, it is required from the standpoint of preventing a four-wave mixing that no zero dispersion wavelength should exist in the wavelength band used, and therefore a non-zero dispersion shifted optical fiber (NZDSF) with no zero dispersion included in the wavelength band used has been developed. Through the development of this NZDSF, WDM transmission has become feasible in a wavelength range of 1530 to 1565 nm (C band) and a wavelength range of 1565 nm to 1625 nm (L band), which has increased a transmission capacity drastically.
In order to increase the transmission capacity in such a WDM optical transmission system, an attempt is made to expand a wavelength bandwidth of transmission signals.
The invention disclosed in U.S. Pat. No. 6,205,268 maintains substantially the same fiber parameters as those of a standard single mode optical fiber as shown in a loss curve 132 and dispersion curve 131 in FIG. 13, reduces a loss peak (133 in FIG. 13) by OH absorption of 1383 nm, reduces a dispersion value of a wavelength band of 1.4 μm and thereby realizes a CWDM (Coarse Wavelength Division Multiplexing) system in a wide wavelength range of wavelength bands of 1.3 μm, 1.4 μm and 1.5 μm. In this CWDM transmission system, the optical fiber has a zero dispersion wavelength in the vicinity of 1310 nm (dispersion curve 131), and therefore transmission using the wavelength band of 1.3 μm for analog CATV transmission and the wavelength band of 1.4 μm for transmission at 10 Gbps or above is proposed. Furthermore, with the proposal of this new CWDM transmission system, a transmission apparatus for transmission in the wavelength band of 1.4 μm has also been developed in recent years and being put to practical use.
With consideration given to the application of WDM transmission to a metropolitan system, given the fact that an overwhelming majority of transmission paths running today are standard single mode fibers, the proposal of above described U.S. Pat. No. 6,205,268 seems excellent. However, given the fact that an overwhelming majority of transmission apparatuses already put to practical use are also transmission apparatuses for the wavelength band of 1.3 μm, it is desirable to use not only the wavelength band of 1.4 μm but also the wavelength band of 1.3 μm for WDM transmission from the standpoints of cost as well as consistency with the actual system.
On the other hand, as the invention disclosed in U.S. Pat. No. 5,905,838, there is a proposal on an optical fiber which shifts the zero-dispersion wavelength to 1350 to 1450 nm as shown in the dispersion curve 134 in FIG. 13 and sets an absolute value of dispersion of 1310 nm and 1550 nm to 1.0 to 8.0 ps/nm/km to thereby realize WDM transmission using both wavelength bands. However, attempting to realize WDM transmission using both wavelength bands results in an unavoidable reduction of the mode field diameter MFD (or effective core area Aeff) as described in the aforementioned US Patent. The above described US Patent regards 49 μm2 as an upper limit of Aeff.
Furthermore, U.S. Pat. No. 6,131,415 sets a cladding/core ratio of a core rod to 2.0 to 7.5 to prevent OH groups in an over cladding from spreading into the core during drawing and realize a low OH fiber. However, it is generally known that an absorption peak by OH groups increases after a hydrogen aging test specified by IEC60793-2-50 (first edition 2002-01) Annex C Section C 3.1 is conducted.
Especially when use in a metropolitan system is considered, the following conditions are further required: (1) Many standard single mode optical fibers are already laid and compatibility with these established optical fibers is important. For this reason, it is desirable to make a compatible design with the standard single mode optical fibers, regarding optical fiber parameters such as MFD, cladding diameter, specific refractive index difference and transmission characteristics such as optical transmission loss, dispersion, cutoff wavelength and mechanical characteristics against bending and lateral pressure etc. (2) Optical fibers are generally formed into a cable and laid in underground conduits. In the case of a metropolitan system, conduits are tangled in a complicated manner and it is difficult to lay the optical cables in long lengths. For this reason, an average length of a cable piece is about 1 km. On the other hand, optical fibers are shipped in piece lengths of 25 to 50 km. Normally, an absorption loss characteristic of 1383 nm by OH groups does not change by cabling, and therefore uniformity in the longitudinal direction of the transmission characteristic of an optical fiber is an important factor to secure the quality of the cable.
In the case of a metropolitan system, multi-core cables having 1000 cores are put to practical use and it is more important for the optical fiber to have excellent uniformity in the characteristic (transmission loss) of approximately 1 km, small loss in connections between fibers, micro bending loss and resistance to lateral pressures, etc., rather than an average transmission loss in long length of 25 to 50 km. From such a standpoint, in above described U.S. Pat. No. 5,905,838, it does not disclose transmission characteristics in short length of the optical fiber and the MFD (Aeff) is as small as approximately 7 μm, and therefore connection loss in a connection with a standard single mode optical fiber having an MFD of approximately 9.2 μm becomes 0.3 dB or above, which is not practical. In this way, attempting to be compatible with existing transmission paths results in not being compatible in terms of transmission apparatuses, and on the contrary attempting to be compatible with existing transmission apparatuses results in not being compatible in terms of transmission paths. Any attempt to optimize this compatibility from both aspects of transmission paths and transmission apparatuses has not been made so far.